1. Field of the Invention
The present invention relates to a thin-film transistor panel having a structure that suppresses characteristic shifts and to a method for manufacturing the thin-film transistor panel.
2. Description of the Related Art
A thin-film transistor panel is used in, for example, an active matrix type liquid crystal display device. The thin film transistor panel has a plurality of pixel electrodes and switching thin-film transistors respectively connected to the pixel electrode. An opposite electrode panel having color filters and an opposite electrode is placed so that a liquid crystal is sandwiched between the opposite electrode panel and the thin-film transistor panel. A display voltage corresponding to the display pixel is applied to between each of the pixel electrodes and the opposite electrode. This changes the transmittance of the liquid crystal to allow an image to be viewed. Jpn. Pat. Appln. KOKAI Publication No. 2005-93460 describes an exemplary structure of a thin-film transistor of the thin-film transistor panel. The thin-film transistor described in the JP 2005-93460 has a gate electrode provided on a top surface of a glass substrate and a gate insulating film provided on top surfaces of the gate electrode and substrate. A semiconductor thin film made of intrinsic amorphous silicon is provided on a top surface part of the gate insulating film which corresponds to the gate electrode. A channel protective film made of silicon nitride is provided in a predetermined area of a top surface of the semiconductor thin film. Ohmic contact layers made of n-type amorphous silicon are provided on the opposite sides of the top surface of the channel protective film and on the top surface of the semiconductor thin film. Source and drain electrodes are formed on top surfaces of the respective ohmic contact layers. An overcoat film made of silicon nitride is provided on the source and drain electrodes.
In the above conventional thin-film transistor, the source and drain electrodes are wider than each ohmic contact layer in an area provided directly on the semiconductor thin film. Further, each ohmic contact layer in the area provided directly on the semiconductor thin film is entirely covered with the source and drain electrodes. Thus, even though the overcoat film made of silicon nitride is formed on the source and drain electrodes by a plasma CVD process, the surface of each ohmic contact layer in the area provided directly on the semiconductor thin film is not damaged by plasma. This in turn makes it possible to avoid shifting a Vg (gate voltage)-Id (drain current) characteristic to a minus side.
However, in the above conventional thin-film transistor, the source and drain electrodes are wider than each ohmic contact layer in the area provided directly on the semiconductor thin film. Consequently, a photolithography process for forming the source and drain electrodes is different from a photolithography process for forming the ohmic contact layers. This increases the number of photolithographic steps.